virttest.utils_net: fix remote runner error

If the runner is remote then runner(cmd) will return command status and result, and we only want to find match text in stdout, so it should be runner(cmd).stdout for remote runner.
Hi @chuanchang  not sure how you using this. If we send the ShellSession functions(like cmd_output etc) which is from aexpect.py to this function as runner. It will return the cmd output strictly. And your modification will cause problems.